Formaldehyde has conventionally been produced on an industrial scale by oxidizing methanol in the presence of oxidation catalysts such as silver or iron-molybdenum catalyst.
A process has also been proposed in which formaldehyde is produced by oxidizing methylal in the presence of methanol oxidation catalysts of the types described above (U.S. Pat. No. 2,467,223).
A method is also known that produces methylal by reaction between methanol and formaldehyde as described in JP-B-40-15005 (the term "JP-B" as used herein means an "examined Japanese patent publication").
Conventional methods of producing formaldehyde by oxidation of methanol have low utilization of methanol due to the formation of a large quantity of water during the reaction. In addition, the maximum concentration of the aqueous formaldehyde solution that can be thus produced is only 55 wt % and it has been difficult to obtain higher concentrations.
The formaldehyde produced by these methods is used in the manufacture of polyacetal resins, formaldehyde resins, phenol-formaldehyde resins, etc. or as a starting material for the production of paraformaldehyde, urea compounds, etc. In these applications, the concentration of formaldehyde (in the case of formalin, the concentration of formaldehyde in aqueous solution) must be as high as possible, namely, the concentration of the product formaldehyde must be increased. To meet this need, various methods have been employed including distillation or scrubbing, or alternatively, formaldehyde is first reacted with an alcohol to form hemiacetal, which then is evaporated by heating to obtain a formaldehyde-rich gas.
When highly concentrated formaldehyde is to be obtained, formaldehyde of low concentration (e.g. formaldehyde of high water content) produced by conventional means is usually unsuitable for use unless it is re-concentrated by distillation or an other appropriate method. The comparatively rigorous conditions of distillation for concentration of dilute formaldehyde to a useful level requires substantial energy consumption. In addition, distillation towers and other units in the system have to be made of special corrosion-resistant materials.
In some instances, formaldehyde is converted to derivatives thereof before it is put to the various use mentioned above. In the case of manufacturing polyacetal copolymers, formaldehyde is first converted to trioxane (a derivative of formaldehyde), which is then used in the manufacturing process. However, when formaldehyde is converted to such derivatives, the generation of unreacted formaldehyde components in the process is unavoidable. Unreacted components are typically discharged from the system as formaldehyde of low concentration (e.g. water-rich formaldehyde) and distilled or otherwise separated as described above, thereby producing formaldehyde of a higher concentration for further use. However, problems of high energy consumption and the need to use special corrosion-resistant materials as described above remain unsolved.
A similar problem occurs in the process proposed in U.S. Pat. No. 2,467,223 for producing formaldehyde by oxidizing methylal. The resulting formaldehyde is of low concentration and must be enriched before it can be of practical use.
It should also be noted that neither U.S. Pat. No. 2,467,223 nor JP-B-40-15005 discloses or suggests a one-step process for producing formaldehyde from methanol by combining a process for producing methylal from methanol and formaldehyde with a process for producing formaldehyde by oxidation of methylal. Neither patent refers to a technique for effectively recycling the dilute formaldehyde (formaldehyde of low concentration) which occurs in the recovery of highly concentrated formaldehyde or derivatives thereof.